Gyros have been used to measure rotation rates or changes in angular velocity about an axis of rotation. A basic conventional fiber optic gyro (FOG) includes a light source, a beam generating device, and a coil of optical fiber coupled to the beam generating device that encircles an area. The beam generating device transmits light beams into the coil that propagate in a clockwise (CW) direction and a counter-clockwise (CCW) direction along the core of the optical fiber. Many FOGs utilize glass-based optical fibers that conduct light along a solid glass core of the fiber. The two counter-propagating (e.g., CW and CCW) beams experience different pathlengths while propagating around a rotating closed optical path, and the difference in the two pathlengths is proportional to the rotational rate that is normal to the enclosed area.
In a resonator fiber optic gyro (RFOG), the counter-propagating light beams are desirably monochromatic (e.g., in a single frequency) and circulate through multiple turns of the fiber optic coil and for multiple passes through the coil using a device that redirects light that has passed through the coil back into the coil again (i.e., circulates the light) such as a fiber coupler. The beam generating device modulates or shifts the frequencies of each of the counter-propagating light beams so that the resonance frequencies of the resonant coil may be observed. The resonance frequencies for each of the CW and CCW paths through the coil are based on a constructive interference condition such that all light-waves having traversed the coil a different number of times interfere constructively at any point in the coil. As a result of this constructive interference, an optical wave having a wavelength is referred to as “on resonance” when the round trip resonator pathlength is equal to an integral number of wavelengths. A rotation about the axis of the coil produces a different pathlength for clockwise and counterclockwise propagation, thus producing a shift between the respective resonance frequencies of the resonator, and the frequency difference, such as may be measured by tuning the CW beam and CCW beam frequencies to match the resonance frequency shift of the closed optical path due to rotation, indicates the rotation rate.
The CW Laser inputs light into the resonator and the CW photodetector detects the CW output of the resonator. The electronics after the CW photodetector controls the CW laser frequency to a resonance frequency of the resonator. The resonance frequency is detected by modulating the laser frequency at f1 and then demodulation the photodetector output at f1. At the resonance frequency the photodetector signal at f1 passes through zero amplitude. The CW integrator controls the laser frequency via the CW laser driver to the resonance frequency by adjusting the laser frequency until the output of the demodulator is zero. The modulation at f1 is electronically summed with the CW integrator output by a summer. The CCW laser is controlled to the CCW resonance frequency in a similar manner, except it is common that the modulation frequency f2 is different than f1 to eliminate errors that arise with light from one direction of propagation in the resonator inadvertently couples into the other direction.
Rotation rate is proportional to the difference between the CW resonance frequency and the CCW resonance frequency. It is common practice that the amplitude of the outputs of the modulation generators are set to maximize the sensitivity of the resonator output to laser frequency deviations from resonance frequency. Most rotation sensing errors are minimized when the modulation amplitude is set at or near the maximum sensitivity. However, rotation sensing errors associated with harmonic distortion most prevalent in 2nd order harmonics of the resonance frequency are simply amplified at the greater sensitivity, doing little to improve the performance of the RFOG.
An example of the interfering effects of the presence of a 2nd order harmonic is evident in FIG. 1. In a plot 3 of the modulation as a function of phase, a pure sine fundamental 5 is added to its 2nd order harmonic 7 to yield a new composite wave 9 that exhibits an apparent asymmetry about the x axis. This asymmetry appears as lower positive excursion in amplitude during the first half cycle and a higher negative excursion in amplitude during the second half cycle. This apparent asymmetry diminishes the accuracy of the RFOG by shifting the measured center frequency of the resonator. Because the difference between the clockwise and counter-clockwise resonance frequencies are the measure of the rotational acceleration in an RFOG, the presence of the 2nd order harmonic degrades the performance of the RFOG.
What is needed in the art is an RFOG that is configured to eliminate or minimize the interfering effects of 2nd order harmonics producing rotational errors.